Oxygen radicals are responsible for a large fraction of the toxic and mutagenic effects of ionizing radiation. Such radicals are also produced by a variety of environmental chemicals and as side products of normal aerobic metabolism. Multilayered cellular defenses have evolved in response to this threat, consisting in part of some enzymes that prevent damage by scavenging reactive oxygen species and others that repair free radical damage to the critical molecular DNA. These defenses are coordinated as inducible systems in Escherichia coli that respond to different kinds of oxidative stress. One group of proteins (the soxR regulon) is induced specifically by the intracellular generation of superoxide radical and increases the levels of several enzymes that destroy superoxide, produce reducing equivalents in the form of NADPH, and repair oxidative damage to DNA. Another set of responses is triggered by hydrogen peroxide, and includes inducible enzymes that destroy organic or hydrogen peroxides, along with a large number of unknown inducible functions. We will dissect the biological functions of these adaptive responses to oxidative stress by a detailed characterization of the soxR gene and its regulon, and by the identification of H2O2-inducible genes through the use of gene fusions. We will determine the DNA damages formed in vivo that require the soxR-controlled enzyme endonuclease IV for their repair. This integrated molecular genetic and biochemical analysis of the biological responses to oxidative damage in a facile bacterial system will provide important information about general biological mechanisms for dealing with the toxicity of intrinsic and radiation-induced oxygen radical damage.